Process for liquefaction of helium by expansion



June 25, 1968 S. ERGENC PROCESS FOR LIQUEFACTION OF HELIUM BY EXPANSION Filed April 2l, 1965 5 Sheets-Sheet l InVenU/T WMV WL MMI s. ERGENC 3,389,565

PROCESS FOR LIQUEFAOTION OF HELIUM BY EXPANSION 5 Sheets-Sheet 2.

June 25, 1968 Filed April 21, 1965 June 25, 1968 s. ERGENC 3,389,565

PROCESS FOR LIQUEFACTION OF HELIUM BY EXPANSION Filed April 2l, 1965 5 Sheets-Sheet 3 Invenar:

Snnnev-Tnv Enea/vc @Wl MM, M), Jalwv M Q4/M2 mw June 25, 1968 s. ERGENC 3,389,565

PROCESS FOR LIQUEFACTION OF' HELIUM BY EXPANSION Filed April 2l, 1965 5 Sheets-Sheet 4 F/lg. 4

.fm/enfer? SMAaTTnv ,faam/c M] uw WLM/W WMA-vd HEL/UM Inventar? June 25, 1968 s. ERGENC PROCESS FOR LIQUEFACTION OF HELIUM BY EXPANSION Filed April 2l, 1965 5 SheetsSheet 5 United States Patent O "ice i 3,389,565 PROCESS FOR LIQUEFACTION OF HELIUM BY EXPANSION Sahabettin Ergenc, Zollikerberg, Zurich, Switzerland,

assigner to Sulzer Brothers Limited, Winterthur, Switzerland, a Swiss company Filed Apr. 21, 1965, Ser. No. 449,744 Claims priority, :application Switzerland, Apr. 29, 1964, 5,628/ 64 9 Claims. (Cl. 62-9) ABSTRACT OF THE DISCLOSURE There is disclosed a process Afor the liquefaction of gases of low boiling point such as helium in which the gas to be liquefied is compressed to supercritical pressure and then cooled below its inversion temperature at least in part by passage in heat exchange relation with previously compressed, cooled and expanded gas. After further cooling the gas is expanded in a first throttling step, without liquefaction, to an intermediate pressure above the liquefaction pressure of the gas. The gas is then divided into two streams. One stream is returned through one or more of the heat exchangers through which the compressed gas flows prior to throttling, and is raised in temperature in the process. This one stream is then passed through an expansion turbine wherein its pressure is lowered to the vicinity of the liquefaction pressure of the gas, whereafter it is heated substantially to room temperature by heat exchange with compressed gas which has not been throttled, being then recycled to the compressor. The other stream of gas, after further cooling, is further expanded in a second throttling step, undergoing partial liquefaction in the process. Unliqueed gas and liquefied gas which undergoes revaporization is combined with that flowing through the expansion turbine for return to the compressor.

The present invention pertains to a process and apparatus for the liquefaction of a gas having a low boiling point, such as helium. In accordance with the invention the gas is first compressed to a high pressure, preferably above its critical pressure. It is then cooled by a heat exchange process and thereupon partially liquefied by'expansion in a throttling step. Processes of this kind are employed either for production of low temperatures or for the purpose of obtaining the liquefied gas as an end product.

It is well known that gases of low boiling point which cannot be cooled by throttling at room temperature must be preliminarily cooled to an intermediate temperature below their inversion temperature in order to permit further cooling of the gas by throttling. It is only after such a preliminary cooling that the so-called Joule-Thomson effect becomes positive for the reason that the enthalpy of the gas at this intermediate temperature and at high pressure, i.e. before throttling, is smaller than the enthalpy of the gas at the same temperature and at low pressure, i.e. after throttling. The Joule-Thomson effect becomes positive if the enthalpy of the gas at high pressure and at the intermediate temperature just referred to is less than the enthalpy of the gas throttled to lower pressure but at the same temperature. The throttling effect, i.e. the difference in enthalpy between the gas stream throttled to lower pressure and the compressed gas at the intermediate temperature, corresponds to the quantity of heat which can be removed from the gas when it is in the range of temperatures from the intermediate temperature downward. This quantity of heat is, in the most `favorable case, equivalent to the amount of thermal energy removed in that range of temperatures 3,389,565 Patented June 25, 1968 example, through introduction of heat from the outside.

Consequently, the magnitude of. the enthalpy difference is determinative for the cooling achieved. The magnitude of the enthalpy difference depends in the first instance on the value of the intermediate temperature with which it is associated and additionally on the pressure values before and after throttling. The enthalpy difference increases with declining temperature and, generally, with increase in difference in pressure between the pressures prior to and after the throttling, as may be Seen from the usual graphic plots such as the temperature-entropy diagrams of, for example, helium.

The increase in enthalpy difference is however not the same, for a given ratio of high to low pressures, at all pressure values. Consequently the energy requirements for compression necessary to achieve Ia given anthalpy change are not the same, for a given pressure ratio, at all values of pressure. The most favorable range of pressure values for a given pressure ratio is on the other hand dependent on temperature, and shifts toward lower pressure ranges with declining temperature.

It is an object of the invention so to dispose that portion of the liquefaction process dependent on the Joule- Thomson effect as to achieve a given cooling effect with lower compression energy requirements than heretofore. Consequently, according to the invention, at least a part of the gas at high pressure (i.e. before throttling) is cooled down to a level below the inversion temperature point. Then, after being further cooled in an exchanger, it is expanded through a throttling valve to an intermediate pressure, the gas so further cooled by expansion remaining however in gaseous form. The gas so partially expanded is then divided into two streams. The first of these is passed as a heat absorber through the heat exchanger immediately upstream of the expansion step and also through at least one additional heat exchanger. In these exchangers the first stream abstracts heat from high pressure gas not having undergone the expansion step, before being aspirated, in the range of ambient temperatures, by a compressor. The second partial stream is subjected to further expansion in a second throttling valve, and the liquefied fraction so obtained is separated 0E. The remainder, including any revaporized gas, passes as a heat absorption medium in heat exchange relation with gas at high pressure until it is rewarmed to the vicinity of ambient temperature.

The quantity of heat, which must be withdrawn from the gas to achieve the said intermediate temperature below the inversion temperature of the gas corresponds to the so-called cooling energy requirement in this range of temperatures, i.e. between the ambient temperature and the said intermediate temperature.

In a preferred embodiment of apparatus according to the invention, a portion of the energy required to be removed from the gas in order to cool it is so removed, after initial compression of the gas, by expansion of the gas in expansion turbines with performance of external work.

In another embodiment of apparatus according to the invention, a portion of the energy required to be removed from a gas for cooling is removed therefrom by exterior sources of cold, in place of the expansion with performance of external work. That is to say., energy is removed lfrom the gas to be cooled by transfer thereof to those sources of cold acting as a heat sink.

According to a further feature .of the invention in a preferred embodiment thereof, the second partial stream is cooled by heat exchange with non-liquefied gas expanded in said second throttling step.

According to a further feature of the invention the first partial stream, after being raised in temperature by heat exchange in at least one exchanger with gas at high pressure, is expanded with performance of work in an expansion stage and is thus brought down approximately to the pressure at which part of the second partial stream is liquefied. With plural-stage expansion of the first partial stream accompanied by the performance of work, the rst partial stream can undergo cooling by heat exchange between two successive stages of such expansion.

Fory extraction from the gas, by expansion thereof with performance of work, of part of the energy required to be removed in order to effect the desired cooling, it can be advantageous to separate out still another partial gas stream beginning at a pressure higher than that to which the first throttling step reduces the pressure of the gas involved therein. This third partial gas stream is then cooled by heat exchange and by expansion with performance of Work, from ambient temperature down approximately to that exhibited by the rst partial stream after being warmed by heat exchange Iwith gas at high pressure which has not undergone the first throttling expansion. This third partial stream is then expanded, together with the gas in the first partial stream, down to the pressure exhibited by the second partial stream after the separate throttling undergone by that second partial stream.

Another mode of achieving part of the necessary energy removal which can be employed is ,one in which part of the high pressure gas is expanded with performance of work down to the intermediate pressure and is then cornbined with the first partial stream after the latter has passed through the heat exchanger immediately upstream of the throttling step for the high pressure gas. The two streams so combined are then raised in temperature subsantially to ambient by heat exchange with high pressure gas and are compressed to high pressure.

A particularly advantageous application of the process of the invention resides in the liquefaction of helium. In this application of the invention it may be desirable to provide that not only the high pressure but also the intermediate pressure of the helium, i.e. that following as well as that preceding the first throttling step, shall be in the supercritical range.

The invention will now be further described in terms of a number of non-limitative examples by reference to the accompanying drawings, in which FIGS. 1 to 5 are schematic diagrams of various forms of apparatus according to the invention suitable for carrying out the process of the invention.

Referring to FIG. 1, the gas to be liquefied is compressed in a compressor 1, preferably to supercritical pressure. The compressed gas then passes through an after cooler 2 for removal of the heat of compression. It then passes through heat exchangers 4, 5, 6 and 7 which may advantageously be of so-called plate-fin type. In these exchangers the gas is cooled by heat exchange with cooler gas flowing counter-current therewith, the operation of these heat exchangers being explained in greater detail hereinafter. The gas so cooled, conveniently called high pressure gas in View of its subsequent reduction in pressure, is thereupon passed through one or the other of two adsorbers 8 and 9 for removal of residual impurities. The adsorbers 8 and 9 are used alternately, the one not in use undergoing regeneration. The high pressure gas has by now been cooled to what has hereinabove been referred to as the intermediate temperature, to be designed TVI, which lies below the inversion temperature of the gas.

The high pressure gas is then further cooled in a heat exchanger 10 and is then expanded by passing through the throttling valve 11 down to an intermediate pressure. The value of this intermediate pressure is so chosen that in being expanded thereto the gas undergoes a cooling, to a temperature TVH below TVI, but no liquefaction. For convenience, the iiow channel from compressor 1 to the low pressure side of throttling valve 11 will be designated 2l. At this latter point is provided a junction 22, from which one line 23 leads back through exchangers 10 and 7 to a junction 24, while another line 25 leads through an exchanger 16 and second throttling valve 17 into a liquid gas reservoir vessel 18.

The gas expanded in valve 11 is divided at junction 22 into two streams. The first of these is led vvia line v23 back through the heat exchanger 1i). In passing through exchanger 10 this stream absorbs heat from the gas flowing from the loutlet end of the adsorbers S and 9 toward the throttling valve 11 and is raised in temperature to the vicinity of the precooling temperature TVI. It then flows on back through the heat exchanger 7 where it also absorbs heat.

At the outlet of the exchanger 7, and more particularly at the junction 24, the first partial stream is combined with another stream owing in a channel 26 from compressor 12 to junction 24. The gas stream flowing in channel 26 has been raised by compressor 12 to a pressure intermediate the inlet and outlet pressures at the throttling valve 11. The gas stream then passes through an after cooler 13 for dissipation of the heat of compression and through the heat exchanger 4 in which it is cooled. It thereupon ows through the expansion turbine 14 and lastly through the exchanger 6 where it undergoes further cooling to a temperature close to that of the first partial gas stream arriving at junction 24. The two gas streams so combined are then passed through expansion turbine 15 in which they are reduced to a pressure close to that of liquefaction for the gas in question. The turbines 14 and 15, and similarly the turbines 44, 45, 46, 47, 48 and 49 of FIGS. 3, 4 and 5, have each coupled thereto a friction brake 14a in which mechanical energy resulting from expansion lof the gas in such turbines may be converted into heat. This heat is however isolated from the gas streams flowing through those turbines. Part of the energy so converted may indeed be reconverted into heat by friction in the bearings of the turbines themselves.

To return now to the output of the throttling valve 11, the second partial gas stream divided off at this outlet passes at a precooling temperature TVH via channel 25 into a heat exchanger 16 in which it is further cooled. Upon emerging from the exchanger 16 it is further expanded in a throttling valve 17 whereupon it undergoes partial liquefaction. The liquefied fraction is captured in a container 18 from which the liquefied product passes through a withdrawal line 19 having a valve 20 therein.

The fraction of the second partial stream not liquefied, and any portion of the liquid gas which has revaporized, passes via a channel 27 back through the exchangers 16 and 10, absorbing heat therein and 4being raised in temperature thereby to a temperature close to the temperature TVI. Upon emerging from the exchange-r 10 this gas is combined at a junction Z8 with that which has passed from junction 24 via line 29 through the expansion turbine 15. The combined streams of channels 27 and 29 then pass via a channel Sti through the exchangers 7, 6, 5 and 4 in that order, in all `of `which they absorb heat from the gas at high pressure in channel 21, and are in fact warmed to the vicinity of ambient temperature. 'From exchanger 4 they flow back to the input side of the compressors 1 and 12. A quantity of make-up gas corresponding to that actually withdrawn `by liquef-action is introduced through an inlet line 3 on the low pressure side of the compressor '1.

There will now be considered a numerical example of the operation of the invention in the liquefaction of helium. The helium may be compressed in compressor 1 to a pressure of approximately 18 atmospheres, and it may be reduced at the throttling valve 11 to an intermediate pressure of about 3 atmospheres, which latter pressure it will be noted is still above the critical pressure for helium. Throughout, atmospheres means atmospheres absolute. In the compressor 12 helium is compressed to about 7 atmospheres and it is expanded in the turbine 14 to about 3 atmospheres, and in the turbine l5 to a pressure somewhat below the liquefaction pressure of helium, which is about 1.25 atmospheres. The intermediate temperature TVI amounts in this example to about y6" Kelvin and the temperature TVH downstream of the first throttling valve 11 is about 5.4" Kelvin, whereas the liquefaction temperature in the container 18 amounts for the assumed pressure of 1.25 atmospheres to some 4.5a Kelvin.

In the embodiment according to FIG. 2, a 'portion of the energy required to be removed from the gas to effect cooling is delivered by the gas to exterior sources of cold, in place of the expansion of the gas with performance of external work which is employed in the embodiment of FIG. l. The elements in the system of FIG. 2 corresponding to those of FIG. 1 are identified with similar reference characters. The embodiment of 'FIG. 2 is likewise adapted to the liquefaction of helium.

A vessel containing liquid nitrogen and a vessel 31 containing "liquid hydrogen are provided as sources of cold or low temperature. Liquid nitrogen is supplied to the container 30` via line 32. Vaporized nitrogen passes from the container 3d through a heat exchanger 33, absorbing heat therein, and thence via a line 34 out of the system. The vessel 31 is fed with liquid hydrogen through aline 35, and from this vessel vaporized hydrogen passes through heat exchangers 36 and 33, also absorbing heat therein, and thence via a line 37 out of the system.

In order t-o cool the helium to the initial intermediate temperature TVI, the helium compressed in the compressor 1 is, after dissipation of its heat of compression in an after cooler 2, passed through the heat exchanger 33.

It is there cooled by heat exchange with vaporized hydron gen and nitrogen Iflowing in channels 34 and 37, and with vaporized helium emerging from the liquefaction vessel 18 and fiowing through channel 51. It is also cooled by heat exchange with the gas of the first -partial stream flowing through channel 23. The high pressure gas, not having undergone expansion in the throttling valve 11, is then passed through a coil 3S in the liquid space ot the nitrogen vessel 30, so that a portion of the nitrogen is vaporized from the consequent heat exchange. The high lpress-ure gas thus further cooled is thereupon -passed through the heat exchanger 36 where it fiows in heat exchange relation with vaporized hydrogen developed in the vessel 31, and with vaporized helium in channel 51 `from the vessel 18 and with the first partial gas stream in channel 23. After purification in one of the adsorbers 8 and 9 it passes through a heat exchange -coil 39l in the liquid space of the hydrogen vessel 31 so that its temperature is reduced to the first intermediate temperature TV1. The remaining steps of the process so far as corresponding to that of the embodiment of FIG. l will not be restated. In departure from FIG. 1 however the first partial gas stream, beginning at the intermediate pressure at the outlet valve 11, is not thereafter expanded with performance of work. Rather, after heating in the exchangers 1f), 36 and 33, it is returned to channel 211 downstream of the first stage 1a of the compressor' and upstream of the second stage 1.

The fraction of the second partial stream not liquefied, together with revaporized helium from the vessel 18, passes through channel 51 and is thereby heated in the exchangers 16, 10, 36 and 33 approximately to ambient temperature, and is thenpassed to the low pressure side of the first stage 1a `of the compressor. The after cooler for dissipation of compressor heat downstream of the first stage ofthe compressor is identified by reference 2a.

According to the embodiment of FIG. 3, the gas is raised to the selected high pressure in a single stage compressor 1. After precooling in an after cooler 2 it is reduced in temperature to the first intermediate `or precooling temperature T VI in exchangers 40, 41 and '42 of channel 21, where it passes in heat exchange relation with a first partial 4stream owing in a channel 53. The gas in channel 21 is then further cooled7 in exchanger 43, by the counter-flowing gas in channel 53, and is then reduced in v the throttling valve 11. The first partial stream is fed Iback by channel 53 through the exchangers 43, 42 and 41 and is heated thereby. It is then expanded in the expansion turbine 44. The gas so cooled in turbine 44 is further cooled in exchanger 41 and is lastly reduced in pressure by expansion in expansion turbine 45 to a pressure below the liquefaction pressure in the vessel 1S. The gas vaporizing out of the vessel 18 flows through a channel 54 where it is heated substantially to the initial intermediate temperature TVI in heat exchangers 16 and 43, this Ibeing the temperature of the high pressure gas in channel 21 at its outlet from exchanger 42. The gas in channel 54 is then combined at a junction S5 with the `gas expanded in the expansion turbine 4S and is passed via a channel 55 through the exchangers 42, 41 and 4d in which it is raised to ambient temperature before being supplied to the input side of compressor 1.

In this embodiment the cooling energy requirement corresponding to passage from the initial intermediate temperature TVI up to ambient, i.e. the thermal energy required to be removed fro-m the gas, is supplied essentially by expansion of the first partial stream with performance of work.

FIG. 4 illustrates a further embodiment of the invention constituting a variant on the embodiment of FIG. 3 as regards the initial cooling of the gas to be liquefied from ambient down to the first intermediate temperature TV1. In correspondence with FIG. 3 the four heat exchangers upstream of the first throttling ofI the high pressure gas are identified with reference characters 4t) to 43. In departure from the embodiment of FIG. 3 however, the cooling load to be met in reducing the temperature of the gas to be liquefied from ambient to the first intermediate temperature TVI is supplied by expansion with performance of work of a portion of the high pressure gas compressed in compressor 1. In particular, a portion of the gas compressed in compressor 1 and cooled in the exchanger 4t) down below ambient temperature is split off from channel 21 at a junction 53 and sent through a channel 59, wherein it is expanded in an expansion turbine 46 approximately to the intermediate pressure of the tirst partial stream in channel 60. It is then further cooled in the exchanger 41 approximately to the temperature of the first partial stream upon the emergence of the latter from exchanger 42. It is thereupon combined with that first partial stream at a junction 61 of the channels 59 and 60,

The gas streams so combined are thereupon expanded in an expansion turbine 47 substantially to the liquefaction pressure of the gas in the container 18 and are combined at a junction 62 with gas vaporizing out of that container through a channel 63, after heating of the latter in the exchangers 16 and 43 substantially to the precooling temperature TVI. The gas streams combining at junction 62 are fed together to the compressor 1 through a channel 64, in which they are raised substantially to ambient temperature in the exchangers 42, 41 and 40.

In a fifth embodiment of the invention illustrated in FIG. 5, the cooling from ambient down to the first intermediate temperature TVI is, as in the embodiment of FIG. 4, effected by expansion, with the performance of work, of a portion of the high pressure gas. This gas is in the embodiment of FIG. 5 compressed in a two-stage cornpressor. The fraction of the gas led off downstream of the exchanger 443 is passed through two successive stages of expansion with performance of work in the turbines 48 and 49 and is thereby brought down to the intermediate pressure characterizing the high pressure gas after initial throttling and expansion thereof. While expansion turbines have been shown, piston machines may be used to the same end. Between the two expansions in turbines 48 and 49 the gas is cooled in a heat exchanger 41. The gas so expanded, which has been reduced in temperature approximately to the initial lintermediate temperature TVI, is combined at a junction 65 with the .first partial stream upstream of exchanger i3 and is conducted together with this partial stream through the exchangers 42, 41 and 40 to the suction side of compressor 1.

According to the invention therefore the gas to be liquefied is first compressed. It is then initially cooled to a first intermediate temperature TVI below the inversion temperature of the gas. This initial cooling is effected by expansion of the gas with performance of external work in expansion turbines, or by cooling with the aid of external cooling means to which heat of the gas is transferred by conduction or convection. The gas is then further cooled by heat exchange and is thereafter throttled to a rst intermediate pressure higher than that of liquefaction. By this first throttling there is achieved for the gas a second intermediate temperature TVH which is however above the liquefaction temperature of the gas. A fraction of the gas thus cooled to the second intermediate temperature is then subjected to a second throttling step in the course of which it is at least partially liquefied. The cooling effected on the basis of the Joule-Thomson effect increases, the lower temperature at which it is carried out, as has been already stated. This means that for the achievement of a given cooling, down to the lowest temperature required to be achieved, there is required a smaller expenditure of energy in compression of the gas than would be the case if the gas were throttled directly from the first intermediate temperature to the liquefaction pressure thereof in order to achieve such liquefaction. It is moreover an advantage of those embodiments of the invention illustrated in FIGS. l, 3 and 5 and which employ expansion turbines for cooling of the gas, that the thermodynamic losses of such turbines are lower at higher temperature ranges than at lower temperature ranges.

It will thus be seen that the invention provides a process for the liquefaction of a gas of low boiling point, such as helium, comprising the steps of compressing the gas to be liquefied to a high pressure preferably above critical, cooling the compressed gas to a temperature below its inversion temperature, further cooling the gas, expanding the gas without liquefaction thereof in a first throttling step, dividing the gas so expanded into two partial streams, passing a first one of those streams in heat exchange relation with the compressed gas prior to the first throttling step for abstraction of heat therefrom and thereafter recycling the gas in the first stream together with make-up gas, expanding the second stream with partial liquefaction thereof in a second throttling step, and returning the fraction of the gas subjected to the second throttling step and remaining in gaseous condition in counter-current fiow heat exchange relation with the compressed gas prior to said first throttling step. The invention also provides apparatus for the liquefaction of such a gas comprising means to compress the gas to be liquefied to a high pressure preferably above critical, means to cool the compressed gas to a temperature below the inversion temperature thereof, means to further cool the gas, means to throttle the gas so further cooled, means to conduct a part of the gas so throttled in counter-current -fiow heat exchange relation with the gas so compressed, means to subject to a second throttling a part of the gas so throttled, and means to conduct the gas remaining in a gaseous state after the second throttling in counter-current fiow heat exchange relation with the compressed gas.

The invention comprehends not only the processes and apparatus hreinabove described, but also all variations thereof and departures therefrom falling within the spirit and scope of the appended claims.

I claim:

1. A process for the liquefaction of helium gas comprising ythe steps of compressing the gas to be liquefied, cooling th-e compressed gas to a temperature below its inversion temperature, expanding all of the gas so cooled without liquefaction of any part thereof in a first throttling step, dividing the gas so expanded into two partial streams, passing a first one of said partial streams in heat exchange relation with the compressed gas prior to said first throttling step for abstraction of heat from such compressed gas, expanding said second partial stream of gas in a second throttling step, and returning the fraction of the gas subjected to said second throttling step remaining in gaseous condition in heat exchange relation With .the compressed gas prior to said rst throttling step for abstraction of heat from such compressed gas.

2. Process according to claim 1 including the further step of subjecting the second partial gas stream, prior to the second throttling thereof, to cooling by heat exchange with nonliquified gas expanded in said second throttling step.

3. Process according to claim 1 including the further step, after passage of the first partial gas stream through at least one stage of heat exchange with said compressed gas, of expanding the gas in the first partial stream with performance of external work substantially to the pressure at which the second partial stream emerges from the second throttling step.

4. Process according to claim 1 wherein said first partial stream is subjected to plural stage expansion with performance of external work and is cooled between successive stages of expansion by heat exchange with the non-liquefied gas expanded in said second throttling step and with the first partial stream before said first partial stream is subjected to expansion with performance of external work.

5. Process according to claim 1 comprising the further step of compressing additional gas to a pressure below that of said compressed gas to be liquefied but higher than that to which said compressed gas is reduced in said first throttling step, and cooling said additional gas by heat exchange and by expansion with performance of external Work substantially to the pressure of the gas in the first partial stream after initial heating thereof by heat exchange with said compressed gas, combining said additional gas with the gas in said first partial stream after such initial heating thereof, and expanding said combined gases with performance of external work substantially to the pressure of the gas in said second partial stream after said second throttling step.

6. Process according to claim 1 including the step of expanding a portion of the compressed gas with performance of externa-l work, combining the so-expanded gas with gas in the first partial stream after initial heating thereof in heat exchange relation with said compressed gas, raising the temperature of ythe socombined gas streams substantially to ambient by passing them in counter-current flow heat exchange relation with said compressed gas, and recycling the combined gas streams so raised in temperature.

7. Process according to claim 1 in which the cooling of the compressed gas is effected at least in part by passing the compressed gas in heat exchange relation with a refrigerating medium other than the gas to be liquefied.

8. A process for the liquefaction of helium gas comprising the steps of compressing .the gas to be liquefied, cooling the compressed gas to a temperature below its inversion temperature, further cooling the gas, expanding all of the gas so cooled and further cooled without liquefaction of any part thereof in a first .throttling step, dividing the gas so expanded into two partial streams, passing a first one of said partial streams through plural stages of counter-current flow heat exchange relation with the compressed gas prior to said first throttling step for absorption of heat from such compressed gas and thereafter recycling the gas in said first partial stream together with make-up gas, expanding said second partial stream with partial liquefaction thereof in a second throttling step, and returning the fraction of the gas subjected to said second throttling step remaining in gaseous condition in counter-current flow heat exchange relation with the compressed gas prior to said first throttling step for absorption of heat from such compressed gas.

9. A process for the lquefaction of helium gas comprising the steps of compressing the gas to supercritical pressure, cooling the compressed gas below its inversion temperature by passing it in heat exchange relation with previously compressed and cooled gas, expanding the cooled gas to a pressure above the liquefaction pressure thereof in a irst throttling step without liquefaction of any part of .the gas so expanded, dividing the gas so expanded into two streams, passing one of said streams in heat exchange relation with compressed gas, thereafter expanding said one stream with performance of external Work down to the vicinity of the liquefaction pressure of the gas, thereafter heating said one stream substantially to ambient temperature by passage in heat exchange relation with compressed gas, recycling to said compression step the gas in said one stream so heated, further expand- 10 ing the other of said two streams in a second throttling step with partial liquefaction thereof, and heating the gas in said second stream not liquefied upon said second throttling step substantially to ambient temperature by passing it in heat exchange relation with compressed gas.

References Cited UNITED STATES PATENTS 2,895,303 7/ 1959 Streeter.

2,957,318 6/ 1956 Morrison.

3,095,274 6/ 1963 Crawford.

3,182,461 5/1965 Johanson 62-23 X NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner. 

